As shown in FIG. 9, conventionally, when manufacturing semiconductors to form an epitaxial thin film by the MOCVD method, carrier gas G1 such as H2 is fed into a pool of raw material 4 in a source tank 5. The liquid-state raw material 4 is stirred by foaming and the raw material 4 is brought into contact with the carrier gas G1 which, at a predetermined temperature, promotes the generation of saturated vapor G4 from the raw material 4. Mixed gas G0, made up of the saturated vapor G4 from the raw material 4 combined with the carrier gas G1, is fed into process chamber 11. FIG. 9 shows a liquid-state raw material (for example, an organometallic compound) 4 pooled inside the source tank 5. However, it is also common to use a solid raw material 4 carried by a porous supporting member located inside the source tank 5 and gas (raw material vapor G4) sublimated from the solid raw material 4.
In FIG. 9, reference numeral G1 represents a carrier gas; G4 represents a saturated vapor of a raw material; G0 represents a mixed gas; 1 designates a carrier gas source (container) such as hydrogen; 2 designates a decompressor; 3 designates a mass flow control device; 4 designates a raw material; 5 designates a source tank; 6 designates a constant temperature reservoir; 7 designates an inlet valve; 8 designates a lead-in tube; 9 designates an outlet valve; 10 designates a valve; 11 designates a process chamber (a crystal growth furnace); 12 designates a heater; 13 designates a substrate; and 14 designates a vacuum pump.
In the apparatus shown in FIG. 9, using a system in which pressure control of the temperature of the constant temperature reservoir 6 (which controls the saturated vapor G4 of the raw material 4) and flow rate control of the carrier gas G1 are superimposed allows the supply amount of the raw material 4 (an epitaxial raw material) to the process chamber 11 to be controlled. However, it proves difficult to highly accurately control the mixture ratio between gases G1 and G4 or the supply flow rate of the mixed gas G0. As a result, variation may easily occur in film thickness or film composition, and thus, the properties of the manufactured semiconductor are not stabilized, which results in the drawback of a lack of consistency.
In the evaporation supply apparatus shown in FIG. 9, in order to achieve highly accurate flow rate control of the mixed gas G0, the pressure of the vapor G0 inside the source tank 5 must be held at a constant set value at all times. However, in the conventional evaporation supply apparatus shown, in the supply system that supplies mixed gas to process chamber 11, only valves 9 and 10 are included, and little consideration is given to this point of the process. As a result, the highly accurate control of the supply amount of the mixed gas G0 is significantly difficult.
To solve this problem, as shown in FIG. 10, a control system for the raw material supply amount has been developed in which the supply system of the raw material vapor G4 and the supply system of the carrier gas G1 are completely separated so that control of the supply amount of the raw material vapor G4 and control of the supply amount of the carrier gas G1 are independently performed. This system has already been put into practical use. In FIG. 10, reference numerals 3a and 3b denote mass flow control devices; 12 denotes a heater; 17 denotes an constant air temperature reservoir; and 18 denotes a mixture unit.
In the control system in FIG. 10, use of a plurality of mass flow control devices 3a and 3b results in two flow rate control systems. Thus, the evaporation supply apparatus is complicated and large, and obtaining a uniform mixture of carrier gas G1 and vapor G4 in the mixture unit 18 is difficult. As a result, it has gradually become apparent that in terms of stability of the properties of a manufactured semiconductor, there is no significant difference between the evaporation supply apparatuses shown in FIG. 9 and FIG. 10.
Further, in recent years, in this type of evaporation supply apparatus, in order to implement a further downsizing of the apparatus and an increase in raw material supply amount, there has been a strong demand to increase the pressure of the raw material vapor G4 and to more highly accurately control the mixture ratio between the carrier gas G1 and the vapor G4 and the supply amount thereof.
In the evaporation supply apparatuses shown in FIG. 9 and FIG. 10, it is difficult to respond to the demand to downsize the evaporation supply apparatus, increase in vapor supply amount, and control of the mixture ratio and the mixed gas supply amount with high accuracy.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2611008    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2611009    [Patent Document 3] Japanese Utility Model Registration No. 2600383